1. Field of the Invention
The present invention relates to a recording apparatus for recording an image on a recording medium.
2. Description of the Related Art
A thermal printer and an inkjet recording apparatus, which records characters and images by ejecting ink onto a recording medium, such as recording paper, are well known as a recording apparatus. An inkjet recording apparatus, which is typically used as an information outputting unit of a printer, a copier, or a facsimile machine, performs recording by ejecting ink while moving a relative position between a recording medium and an inkjet recording head. The image quality of a recording result of the inkjet recording apparatus depends on control of a relative velocity between the inkjet recording head and the recording medium, control of an ejection timing associated therewith, and stability of power supply to the recording head.
Ink jet recording apparatuses are broadly divided into a so-called serial type and full-line type. The serial-type recording apparatus performs recording by ejecting ink while moving an inkjet recording head and is commonly and widely used.
The ink ejecting recording head can eject ink by the operation of a piezoelectric element, the momentary surface boiling of ink, or some other operation. A recording head that ejects ink by boiling of ink supplies ejection energy by boiling ink adjacent to a heater in the vicinity of an ink path adjacent to an ink nozzle by energization of the heater.
In order to maintain satisfactory image quality, it is important to generate uniform ink droplets by continuous stable supply of energy required for ejecting ink and by maintaining the same conditions for ink ejection. However, in recording operations, a duty ratio varies with image data, and thus the number of heaters simultaneously energized varies. Therefore, driving conditions vary depending on effects of voltage variations caused by the difference between output currents of a power source, the difference between drop voltages caused by the resistive component in a transmission system, and some other factor.
Ink ejection control described above is carried out in a range that satisfies a stable ejection condition by use of high accuracy of an output voltage of a power source or a transmission system that has a structure with lower loss. A DC/DC converter that supplies the recording head with power is described below.
FIG. 10 illustrates a block diagram of a voltage control system including a DC/DC converter in the related art. A DC/DC-converter input voltage Vin supplied from a power-source unit (not shown) is input to a switching element 201. The input voltage Vin is output from the switching element 201 and a diode 209 as a direct-current output converted via an inductor 202 and a capacitor 204. The output is supplied as an output voltage VH to a recording head being a load. The switching element 201 is connected to a capacitor 203 at its input side and to the capacitor 204 at its output side via the inductor 202. The inductor 202 and the capacitor 204 constitute a smoothing circuit 205. The output voltage signal VH detected from an output terminal of the smoothing circuit 205 is subjected to direct-current voltage division by a resistor R1 and a resistor R3 in a voltage control circuit 206, is input to an error amplifier 207 included in the voltage control circuit 206, and is feedback-controlled.
An output signal from the error amplifier 207, which receives a Vcc potential from a reference voltage 212 and a feedback output voltage signal VH, is an output signal of the voltage control circuit 206 and performs pulse-width modulation (PWM) control and constant voltage control on the switching element 201 via a PWM comparator circuit 208. A resistor R5 and a capacitor C1 connected to an inversion terminal and an output terminal of the error amplifier 207, respectively, constitutes an example of a phase compensation circuit for adjusting stability and responsivity of an output voltage.
As described above, power is supplied such that feedback is controlled so that a stable output voltage is supplied so as to accommodate variations in output current caused by variations in the number of nozzles simultaneously driven on a recording head being a load.
The recent improvement in semiconductor processes enables the switching speed of a DC/DC converter to be driven at the megahertz level. Additionally, the responsivity of feedback control found in current-mode control is becoming high, like the advent of a control IC that achieves high responsivity on the order of microseconds, so it becomes possible to accurately supply power.
For an inkjet recording head, the performance of ink ejection is apt to depend on the temperature of the recording head. In particular, in a thermal inkjet recording apparatus, since ink droplets are ejected from nozzles by film boiling of ink caused by energization of heater resistors of the recording head, the temperature of the recording head during printing varies constantly.
In particular, when a larger number of sheets are printed continuously or a high-duty image is printed, the frequency of foaming of ink in the recording head increases and the number of ink ejection operations is high, and thus the temperature of the recording head gradually rises.
For the performance of ink ejection with respect to temperature, at low temperatures, the viscosity of ink is high and thus it is difficult to eject, and at high temperatures, the viscosity of ink is low and thus the amount of ink ejected from nozzles increases. Therefore, a defect of ink ejection, such as an increased diameter of a dot of a pixel formed on a recording medium, occurs.
Japanese Patent Laid-Open No. 10-119273 describes correction of energy for heat generation with respect to variations in the temperature of a recording head by controlling a driving pulse width.
However, if the temperature of the recording head itself continues rising by continuous recording operations, control of only the driving pulse width may not suppress an increase in the amount of ink ejection. That is, under present circumstances, it is difficult to perform control of reducing the amount of ink ejection by only pulse-width control.